LED (light emitting diode), is a semiconductor light emitting device, which can emitting light when current passes through. Generally, the LED chips need to be packaged to provide mechanical support, electrical connection, optical guidance, and heat flux path to outside. After that, the LED packages are integrated into systems as a function unit. An LED package generally includes a substrate member on which an LED or LED are mounted. The LED package may include a mounting pad attached to the substrate member with electrical connections being made to the LED for applying an electrical bias. LEDs generally comprise an active region of semiconductor material sandwiched between two oppositely doped layers. When a bias is applied across the doped layers, holes and electrons are injected into the active region where they recombine to generate light. The substrate member may also include traces or metal leads for connecting the package to external circuitry and the substrate may also act as a heat sink to conduct heat away from the LED during operation.
A SMD (surface mounted devices) LED package is one of the LED packages, which electrical connectors come from the underneath of the substrate (see FIG. 1). This type of package has advantages in compact design, low cost, high performance, and more flexible in system integration. The regular materials for the SMD substrate is ceramics, like Al2O3 (Alumina) or AlN. Alumina is chip but with poor thermal conductivity; AlN has good thermal conductivity but very expensive. Therefore, how to balance them is a question.
As the deep penetration of LED into the general lighting market, the higher lumen output is requirement to meet the specifications and compete with other light source, like fluorescence or Halide bulb. High lumen output means high power consumption, and same time, high heat dissipation. The heat dissipation of one SMD package can be as high as 50 to 100 Watts. To keep the LED dies under low temperature, the package thermal performance must be high. Sometimes, even the AlN cannot meet the requirement. Therefore, more advanced materials with higher thermal conductivity is preferred.
To increase light output, sometimes it is preferred to use multiple dies in serial or in parallel in a unit. That will increase the emitting area and therefore light output in a same footprint package.